The purpose of the research is to define and describe aspects of protein movement through the kidney. An unanesthetized rat model is used. A reproducible fraction of the apical pinocytotic vacuole- lysosomal system is recovered in a 10,000 x G pellet from renal cortex homogenate which contains pinocytosed protein. The system can be studied by inducing proteinuria with volume expansion or renal disease, and by directly influencing the system with agents such as vinblastine. Infused bovine albumin enters the system in a predictable manner. Protein clearances and serum protein concentration are determined by a micro solid phase radioimmunoassay. Volume expansion proteinuria is studied by altering serum albumin concentrations (rat and bovine) and rate and volume of infusate.